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Transient Solder Separation of BGA Solder Joint  During Second Reflow Cycle
Transient Solder Separation of BGA Solder Joint During Second Reflow Cycle
This paper covers a study of the mechanism for solder joint separation due to VIPPO and non VIPPO mixed designs.
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Authored By:
Steven Perng and Weidong Xie
Cisco Systems, Inc.

Summary
As the demand for higher routing density and transfer speed increases, Via-In-Pad Plated Over (VIPPO) has become more common on high-end telecommunications products. The interactions of VIPPO with other features used on a PCB such as the traditional dog-bone pad design could induce solder joints to separate during the second and thereafter reflows. The failure has been successfully reproduced, and the typical failure signature of a joint separation has been summarized.

To better understand the solder separation mechanism, this study focuses on designing a test vehicle to address the following three perspectives: PCB material properties, specifically the Z-direction or out-of-plane Coefficient of Thermal Expansion (CTE); PCB thickness and back drill depth; and quantification of the driving force magnitude beyond which the separation is due to occur.

The test vehicle is designed such that each VIPPO pad is surrounded by dog-bone via pads and each of the VIPPO joints has independent daisy chain for in-situ monitoring during the second reflow cycle.

There are four different pad designs: all VIPPO design; VIPPO and dog-bone mixed pad design; VIPPO and skip via mixed pad design; and back-drilled VIPPO mixed pad design. The all VIPPO design is the baseline benchmark. The VIPPO and dog-bone mixed pad design is expected to be the worst case scenario. The VIPPO and skip via mixed pad design together with the VIPPO and back-drilled VIPPO mixed design areincluded to narrow the magnitude of inherent build-in stress induced by the CTE mismatch which causes the VIPPO joints to separate during the second reflow.
The test vehicles are fabricated with two different PCB materials. Material A is a traditional high-end PCB material with high Z-direction (out-of-plane) CTE; while Material B has approximately one third of the Z-direction CTE of Material A.

A Design of Experiment (DoE) with two PCB materials (Material A and Material B) and two PCB thicknesses (93mil and 125mil) has been performed. With the designed single-ball daisy chain test vehicle and installed thermocouples, the correlation between electrical continuity (daisy chain resistance) and solder joint temperature (thermocouple) can be derived.

A video was taken of two cross-sectioned samples during the second reflow cycle using a reflow simulator. The observation is consistent with the findings of the test vehicle (TV) for in-situ monitoring. The results also provide more accurate and broader information for the investigation on why, how, and when the solder separation occurred during the second reflow cycle.

Conclusions
A thorough study of the mechanism for solder joint separation due to VIPPO and non VIPPO mixed designs has been conducted. The unique daisy chain design of the test vehicle enables independent in-situ monitoring of each VIPPO joint that is surrounded by non VIPPO joints. The results of such in-situ monitoring along with a close up video monitoring of the VIPPO joint during second reflow using a reflow simulator provide an in-depth understanding of the solder separation mechanism.

The key findings of this study are:

1. The primary driving force of solder joint separation is the out-of-plane thermal expansion of the PCB which includes two key factors: PCB material out-of-plane CTE and the thickness of PCB (or the depth of back drill);

2. The joint separation occurs before solder liquidus;

3. The separation happens at the interface between VIPPO solder and IMC at the component pad side;

4. The molten solder retouches the separated pad to regain electrical continuity but may or may not reform to be a good quality joint.

In summary, the electrical testing is not effective to identify the separated joints. The issue could be mitigated or even prevented initially ,if feasible, by either selecting PCB materials that have lower out-of-plane CTE and/or extremely high Tg or reducing the thermal expansion linear length such as the use of thinner PCBs.

Initially Published in the IPC Proceedings

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